It's surprisingly easy to dislocate a man's shoulder. Using a move known in Japanese as Oogyaku, or "great reversal," you just have to get him facedown, pull his arm backward and apply pressure to his shoulder with your hand, foot or knee. A small amount of force, and -- snap -- you've popped his arm out of the shoulder joint.

I didn't learn this maneuver at a self-defense or karate class. Instead, I picked it up taking a Ninja Training 101 lesson on Curious.com.

The Menlo Park, Calif.-based e-learning site offers more than 10,000 curated short-form, interactive videos taught by 1,000 teachers on a variety of esoteric topics, ranging from macrame to triathlon training to calculus to the martial arts.

That approach puts Curious.com in the middle of the online-learning spectrum: between YouTube -- with its ocean of video tutorials that may (or may not) teach something useful -- and more formal, classroomlike sites such as Lynda.com and Coursera that focus on academic or professional topics.

The so-called e-learning market is experiencing a boom as people find new ways to cram learning into their already full days. Worldwide revenues in the field are forecast to hit $51.5 billion by 2016, according to a March 2014 study by Docebo (PDF), which builds e-learning management systems for businesses.

More people are seeing the value of online classes than they did before.

Craig Weiss, CEO of E-learning 24/7

"More people are seeing the value of online classes than they did before," said Craig Weiss, CEO of E-learning 24/7, an online-education consultancy. "They're thinking, 'I learn the way I want to learn. Some people are fast, some people are slow. If I make a mistake nobody knows about it, I can go back to that area as often as I want to.'"

Curious.com offers classes in eight main categories, including food; technology and business; language; healthy and fit; brainy; and DIY, and roughly 20 subcategories. Costs range from free to roughly $200. Since the company's inception, it says more than 3.5 million lessons have been viewed.

I decided to try out a few classes myself. Along with the $14.99 Ninja Training 101 class, I signed up for the free Greeting Phrases in Arabic course and the $34.99, 13-lesson course called "DSLR photography for beginners."

Though I'm quite handy with my vintage film camera and my smartphone's camera, my high-end DSLR has always been a mystery to me. In my Curious.com photo class, the instructor walked me through the basics of using a DSLR, explained the camera's menu settings and dived into the more complex issues of composition, depth of field, white balance, shutter speed, aperture, exposure and flash. More than 1,300 students have enrolled in this class.

Other classes also pull in their fair share of students. Seamstress and fashion blogger Mimi G has attracted more than 4,000 enrollees to her courses; Sunset Magazine's home and cooking tips have pulled in nearly 4,500 students. And 28,000 people have enrolled in Motion Training's tips on Microsoft Office.

The O'Neil Sisters filming one of their instructional crafting videos for Curious.com.
The O'Neil Sisters

Jennifer and Kitty O'Neil, dubbed the O'Neil Sisters, are among the site's most popular teachers. The pair -- who spend three to four days mapping out, shooting and compiling each lesson -- have been with Curious since its 2013 launch. In that time, they've attracted more than 15,000 students, who've taken a combined 97,000 lessons on do-it-yourself crafting, such as necklace beading, mosaic building and wood-chair refinishing.

"It's more targeted than just throwing a video on the Internet," said Jennifer, explaining the appeal of her classes. "We are engaged with our students."

That engagement is by design -- it's a way for Curious to differentiate itself from YouTube's mountain of how-to's, which sometimes stop short of actually teaching people what they want to learn.

Justin Kitch felt that frustration with YouTube in 2010. Kitch always wanted to amp up his guitar skills to be able to play gigs. But his work -- first as founder of website-hosting platform Homestead and next as Intuit's chief growth officer -- consumed his days. That summer, however, Kitch decided to take a year off, spend time with his family and hone his guitar playing.

"I set off to learn," he said. "I went to YouTube but I was really underwhelmed [by the quality] and confused." A big problem: No way to interact or get feedback from YouTube's lesson instructors.

So he decided to do something about it. In 2012, he co-founded Curious.com and became its CEO.

The company vets each lesson before publishing it. It helps teachers by giving them a fill-in-the-blank interface to upload lectures, while also promoting their classes. For its service, the company takes roughly a 30 percent cut of the cost of each class.

The site makes it easy for students to send and receive feedback. It also helps them suss out the best teachers and classes by displaying comments from students, along with data showing how many people have enrolled in each class. There's also the "Love" button on lesson pages, which lets viewers easily give positive feedback (in the same way Facebook users can "Like" posts).

I used that Love button and students' reviews to sign up for my Ninja class.

I've watched my sensei lay out his volunteer with the Oogyaku shoulder attack move about a dozen times now. Though I've yet to master the move, I can just tap replay on my computer and keep trying.